Measurement of the $\tau$ lepton polarization and $R(D^*)$ in the decay $\bar{B} \to D^* \tau^- \bar{\nu}_\tau$

We report the first measurement of the τ lepton polarization Pτ(D*) in the decay B¯→D*τ-ν¯τ as well as a new measurement of the ratio of the branching fractions R(D*)=B(B¯→D*τ-ν¯τ)/B(B¯→D*ℓ-ν¯ℓ), where ℓ- denotes an electron or a muon, and the τ is reconstructed in the modes τ-→π-ντ and τ-→ρ-ντ. We use the full data sample of 772×106 BB¯ pairs recorded with the Belle detector at the KEKB electron-positron collider. Our results, Pτ(D*)=-0.38±0.51(stat)-0.16+0.21(syst) and R(D*)=0.270±0.035(stat)-0.025+0.028(syst), are consistent with the theoretical predictions of the standard model.

Note:

7 pages, 2 figures, submitted to Physical Review Letters; one figure was removed from the first version

B: pair production
lepton: polarization
tau: polarization
tau: hadronic decay
anti-B0: semileptonic decay
branching ratio: ratio: measured
electron positron: annihilation
electron positron: colliding beams
KEK-B
BELLE

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